1. Field of the Invention
The present invention relates to a ceramic lid for sealing a semiconductor element and a method of manufacturing the same. Specifically, the present invention relates to an alumina ceramic lid having excellent sealing strength with a ceramic package for housing a semiconductor element and a method of manufacturing the same. More specifically, the present invention relates to a ceramic lid for sealing an EPROM (Erasable and Programmable Read Only Memory) semiconductor element for erasing once-stored memory by ultraviolet radiation.
2. Related Art Statement
Hitherto, as a ceramic package for an EPROM semiconductor element, as described in Japanese Pat. Laid-open No. Sho 61-194,751(1986), there has been known such a structure that an EPROM semiconductor element is mounted on a recess of a ceramic package, and said package is hermetically sealed to an ultraviolet-transmissible ceramic or glass lid by means of a sealing glass. That is, as shown in FIG. 4, on a ceramic package 11 there is formed a recess 12 on the upper surface and a semiconductor element 13 and a lead frame 14 are secured in the recess by glass layers 15, 16, respectively. The semiconductor element 13 is bonded to the lead frame 14 by means of a wire 17. On an upper surface of the ceramic package 11 is sealed a plane ceramic or glass lid 18 via a glass layer 15 so as to seal the recess 12.
Moreover, as shown in FIG. 5, such a structure is also known that a bottom portion 16, a wire bonding portion 20 and a conductor portion 19 of a recess 12 of a ceramic package 11 are metallized, respectively. To both end portions of the conductor portion 19 lead frames 14 are brazed. Ceramic frames 21 for supporting a lid 18 are laminated on the upper surface of the conductor portion 19, and a ceramic or glass lid 18 is soldered 15 on glass-sealed or the upper surface of said ceramic frames 21.
As a ceramic package, use is made of alumina ceramics viewed from electrical properties such as thermal expansion, thermal conductivity, insulation resistance and the like, and mechanical strength.
Moreover, as a lid for sealing a semiconductor element, use is made of metal, plastic or ceramics, but when the package is alumina, it is preferable to use alumina ceramics so as to thermal expansion. Particularly, in the case of an EPROM semiconductor element, use is made of translucent polycrystalline alumina in relation to ultraviolet ray permeability as disclosed in U.S. Pat. No. 3026210, U.S. Pat. No. 3792142 and Japanese Pat. Laid-open No. Sho 63-242,964(1988).
However, in a ceramic package of the prior structure, a sealant of low melting glass at 400-500.degree. C. is printed and dried on a translucent polycrystalline alumina lid or an alumina lid of high purity of more than 98 wt %, a semiconductor element is mounted within the package, and thereafter the sealant is heated to more than the melting point thereof. Although the ceramic lid is sealed to the ceramic package, the sealing strength is disadvantageously small.
Moreover, for higher data processing and miniaturization of ceramic packages, it is required to increase the memory amount of EPROM semiconductor elements, while the ceramic package for the memory semiconductor should be miniaturized. However, the high density technique of a semiconductor element is limited, and a size of the element becomes large with the increase of memory amount. Moreover, a size of a semiconductor ceramic package for memory is standardized for actually mounting on circuit boards, and it is difficult to change the size with an increase in element size.
An explanation is made in detail as follows. When the size of a ceramic package is 11 mm in width, 20 mm in length and 1.7 mm in thickness (except a lead portion), an ultraviolet irradiation portion of the semiconductor element requires about 9 mm.times.9 mm at 1 Mega bit and about 9 mm.times.14 mm at 2 Mega bit. In this case, the sealing portion of an ultraviolet permeable ceramic lid with a ceramic package becomes about 1 mm at 1 Mega bit and 2 Mega bit in the width direction, and 11 mm and 6 mm, respectively, at 1 Mega bit and 2 Mega bit in the lengthwise direction. That is, a sealing area is 139 sq. mm at 1 Mega bit and 94 sq. mm at 2 Mega bit. Therefore, in case of using a 2 Mega bit EPROM semiconductor element for the prior ceramic package structure, there are such disadvantages that sealing strength between a ceramic lid and a package is weak and an airtight state cannot be maintained.